Fail write_build_config.py if current_toolchain != default_toolchain

There are a few spots in the Android templates that assume appending
__build_config to a given target label will result in the label for a
write_build_config action. When toolchains exist in a label, appending
like this doesn't work. E.g.: :some_label(//toolchain)__build_config

Previously, we added a GN assert to enforce default toolchain, but that
was too strict, as it made it so no other targets that are defined in
the same BUILD.gn file could use a non-default toolchain.

This change make the error a build-time error rather than a gn gen
error.

BUG=629371

Review-Url: https://codereview.chromium.org/2161063003
Cr-Original-Commit-Position: refs/heads/master@{#408576}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 83bcd862038586061feea4b390ac524ab0a9a358

android/gyp/write_build_config.py

@@ -279,11 +279,15 @@ def main(argv):
       help='Path to the proguard .info output for this apk.')
   parser.add_option('--has-alternative-locale-resource', action='store_true',
       help='Whether there is alternative-locale-resource in direct deps')
+  parser.add_option('--fail',
+      help='GYP-list of error message lines to fail with.')
 
   options, args = parser.parse_args(argv)
 
   if args:
     parser.error('No positional arguments should be given.')
+  if options.fail:
+    parser.error('\n'.join(build_utils.ParseGypList(options.fail)))
 
   required_options_map = {
       'java_binary': ['build_config', 'jar_path'],

config/android/internal_rules.gni

@@ -336,6 +336,25 @@ template("write_build_config") {
           rebase_path(invoker.bundled_srcjars, root_build_dir)
       args += [ "--bundled-srcjars=$_rebased_bundled_srcjars" ]
     }
+    if (current_toolchain != default_toolchain) {
+      # This has to be a built-time error rather than a GN assert because many
+      # packages have a mix of java and non-java targets. For example, the
+      # following would fail even though nothing depends on :bar(//baz):
+      #
+      # shared_library("foo") {
+      # }
+      #
+      # android_library("bar") {
+      #   deps = [ ":foo(//baz)" ]
+      #   assert(current_toolchain == default_toolchain)
+      # }
+      _msg = [
+        "Tried to build an Android target in a non-default toolchain.",
+        "target: " + get_label_info(":$target_name", "label_with_toolchain"),
+        "default_toolchain: $default_toolchain",
+      ]
+      args += [ "--fail=$_msg" ]
+    }
   }
 }